Ifc Custom Property set: Double Psets / Property not written to Ifc

Hi @AngelVelezSosa ,

With 20.2.1 I have 2 issues with a custom property set.

1. The Revit parameter 'Classification Code' isn't exported to the custom property 'NL-SfB code'. With IfcBuildingElements this property is filled in the Revit **type** parameter _Assembly code_, while with IfcDistributionElements, this property is filled in the Revit **instance** parameter _Classification code_.
With earlier versions this didn't matter because the IFC exporter checked wich one was filled in, and used that one for the IFC export.

2. The custom property set is shown 2x times in Solibri, while shown 1x time in BIMcollab Zoom but with double propertys. This happens when I'm defining differen custom propertys for different Ifc entitys.


For both issues, see attached files for review.

Regards,
Willem
[Custom Pset file.zip](https://github.com/Autodesk/revit-ifc/files/5666796/Custom.Pset.file.zip)

Start by reviewing the attached Custom Pset file.zip and reproducing the export with Revit 20.2.1, then inspect the resulting IFC in Solibri and BIMcollab Zoom. Done means the Classification Code is exported to NL-SfB code and custom property sets are neither duplicated nor populated with duplicate properties.

Written by the indexing model from the issue text.